Prism, particularly for optical data communication

ABSTRACT

The prism, which is used particularly for optical data communication by means of a modulated light beam, is furnished with a transmissive prism body ( 60 ) which has two triangular first side faces ( 69 ) on its outer side and a base surface ( 77 ) therebetween, and two second side faces rising up from the base surface and inclined toward one another as light incidence and light exit faces ( 74, 76 ). The two second side faces ( 74, 76 ) are inclined oppositely, relative to an axis ( 79 ) running parallel to the base surface ( 77 ) and penetrating the planes in which the first side faces ( 69 ) lie, and each run at an acute angle ( 78, 80 ) to the axis ( 79 ) such that reflection radiation is reflected to the side.

The invention relates to a prism for deflection of electromagneticwaves, and particularly a prism for optical data communication by meansof a modulated light beam.

Prisms for deflection of electromagnetic waves are known in a largevariety of technical fields. In the normal case, a prism comprises atransmissive prism body which in lateral view is triangular. Between thetwo triangular first side faces of the prism body, the latter comprisesa base surface and two second side faces extending up from the basesurface, said second side faces bordering on each other opposite to thebase surface. Normally, these two second side faces are, on the onehand, the light incidence face and, on the other hand, the light exitface of the prism.

In prisms, it is generally known that these can comprise a pyramidalerror. A pyramidal error is defined to denote faulty positions of thebase surface and the side faces (i.e. the light incidence and light exitfaces) of a prism which cause the prism to assume the shape of a plateseparated from out of the prism. (As to the definition of pyramidalerrors, see e.g. NAUMANN, H.; SCHRODER, G.: Bauelemente der Optik. 6thedition, 1992, pp. 180-181, ISBN: 3-446-17036-7). As evidentparticularly from picture 5.7.19 of this publication, the side faces ofthe prism are wrongly inclined in the same sense, notably toward eachother, so that their extensions will converge in upward directions toform the tip of a pyramid.

So-called reflection prisms for deflection of light beams, preferablywith low expenditure for adjustment as compared to mirrors, are knowne.g. from the LINOS catalog of 2005/2006, p. 95.

In order to deflect a light beam with the aid of prism while being ableto select among various deflection angles, there is required a relativemovement between the light incidence face of the prism and the lightbeam. In most cases, this is realized in that the prism is rotatableabout an axis which extends substantially parallel to the base surfaceand penetrates the two first side faces of the prism body.

In dependence on the current constellation in a given situation, casesmay occur where the light beam is incident on the light incidencesurface under a right angle, i.e. vertically, and respectively will exitfrom the light exit surface vertically, i.e. in the direction of thenormal line. In both cases, back reflections will occur on the lightincidence surface and respectively the light exit surface. This can beminimized by an anti-reflective coating or another design of saidsurfaces. However, back reflections cannot be entirely prevented.

Back reflections of the type described above will cause a reduction ofthe energy of the light beam passed through the prism. In situationswhere the light beam is modulated, as will be the case in optical datacommunication, back reflections toward the transmitter will lead todisturbances of the data communication.

It is an object of the invention to provide a prism in which the extentof disturbing back reflection is further reduced.

To achieve the above object, the invention provides a prism,particularly for optical data communication by means of a modulatedlight beam, comprising

-   -   a transmissive prism body which has two triangular first side        faces on its outer side and, between said first side faces, a        base surface and two second side faces rising up from the base        surface and inclined toward one another as light incidence and        light exit faces,    -   said second side faces being inclined oppositely, relative to an        axis running parallel to the base surface and penetrating the        planes in which the first side faces lie, and each running at an        acute angle to the axis in such a manner that reflection        radiation is reflected to the side.

Thus, according to the invention, the prism is provided with second sidefaces which are tilted oppositely and substantially under identicalangles and respectively are laterally inclined, which is to say that thelight incidence face and the light exit face are laterally tiltedoppositely to each other, Thereby, the reflected portion of the incidentlight beam will not travel back exactly in the opposite direction to thedirection of propagation of the light beam but will be deflected underan acute angle oppositely to this direction, and thus will not disturbthe optical system emitting the beam. This is of advantage particularlyin optical data communication which is performed by use of modulatedlight beams. Said lateral tilting in opposite direction has theadvantage that the unavoidably generated lateral deflection upon entryof the light beam into the prism will be compensated again when thelight beam is exiting from the prism, i.e. that there will occur alateral parallel displacement of the beams.

The reduction and respectively elimination of back reflection on thelight incidence side and respectively on the light exit side could beobtained e.g. also in that the direction of propagation of the lightbeam together with that axis which extends parallel to the base surfaceand through the two planes in which the two first side faces of theprism are arranged, forms an angle deviating from a right angle. Inother words, one could tilt the prism on the whole relative to the axisof light incidence. Then, however, it would not be possible anymore toachieve, at the light exit surface, a compensation for the lateraldeflection of the light beam, except for a case where the light exitsurface is tilted oppositely to the angle of inclination between thelight beam and the light incidence surface. The scope of the inventionalso covers a prism design and a relative arrangement with respect tothe direction of the incident light beam wherein no paralleldisplacement will occur.

According to an advantageous embodiment of the invention, it can furtherbe provided that the two second side faces of the prism body, i.e. thelight incidence surface and the light exit surface, are anti-reflectiveand respectively are designed to the effect that they dampenreflections.

According to a further advantageous embodiment of the invention, it isprovided that the respective acute angle of inclination is in the rangefrom a few 1/10 angular degrees up to angular degrees in the lowertwo-digit range, preferably from an angular degree of 0.1 to 15 angulardegrees, and with particular preference from 2 angular degrees to 5angular degrees.

The concept, provided by the invention, of a mutual tilting of the lightincidence surface and the light exit surface against the beam directioncan be applied in prism bodies which in lateral view represent anequilateral right-angled triangle or, however, an irregular triangle.

Thus, to sum up, the underlying problem to be solved by the inventionand the solution provided by he invention can be described as follows:

Depending on the tilting angle of the prism, the light beam which is tobe passed through the prism and in the given case is to be deflected,can impinge vertically onto the light-incidence surface of the prism andrespectively exit vertically from the light exit surface of the prism.Understandably, in the process, there will unavoidably occur backreflections into the transmitter, which is disadvantageous particularlyin the modulated light beams or light bundles used in opticalcommunication technology. Therefore, according to an advantageousembodiment of the invention, it is proposed that the transmissivecomponent of the optical deflection element comprises a light incidencesurface and a light exit surface which, for minimizing the effects ofback reflections of radiation impinging on the light incidence surfaceand/or of radiation exiting through the light exit surface, areoppositely tilted around the tilting axis and each extend at a rightangle relative to the tilting axis, wherein reflection radiation will bereflected to the side,

By the lateral, oppositely directed tilting of the light incidence andlight exit surfaces, it is achieved, on the one hand, that theunavoidably generated reflections of the reflected electromagnetic waveswill not return back into the transmitter but will be reflected to theside and, on the other hand, the opposite directionality of the tiltedconfiguration makes it possible that there will occur only a paralleldisplacement of the beam relative to the aperture but not a tilting ofthe radiation bundle in the tilting direction of the surfaces,

This measure can of course be combined with a design of the lightincidence and light exit surfaces which leads to a reduced reflectionand increased transmission. Such measures are generally known e.g. inthe form of antireflection coatings in prisms.

Depending on the tolerated overall site of the prism, the lateral angleof inclination of the above mentioned surfaces of the prism can assumeany conceivable value so that the angle of inclination is an acuteangle. However, the larger the angle of inclination is, the wider theprism has to become so as to be able to transport light in a quantitycorresponding to the light-beam aperture. In regard to a miniaturizationof the prism and its geometrical dimensions and, thus, in regard to alight-weighted and small-sized design of the prism, it is of advantageif the angle comprises only a few degrees. Here, angles of inclinationfrom e.g. an angular degree of 1/10 to angular degrees in the lowertwo-digit range, particularly from an angular degree of 0.2 to anangular degree of 15 and especially from an angular degree of 0.2 to anangular degree of 5 and respectively 10, are possible.

The prism according to the invention can be used generally in alloptical applications where, already today, use is made of prisms for thedeflection of light beams and respectively the processing of lightbeams. The use of the prism of the invention is of particular advantageof optical data communication and preferably in a so-called CoarsePointing Assembly (CPA) unit as described e.g. in PCT/EP2012/069354, thecontent of which is herewith included in the subject matter of thepresent patent application by way of reference.

The invention will be described in greater detail hereunder by way of anexemplary embodiment and with reference to the drawing. In particular,

FIGS. 1 to 4 show perspective and lateral view of a prism according toan exemplary embodiment of the invention for illustration of the mutualtilting of the light incidence and light exit surfaces.

In FIGS. 1 to 4, a design of the prism of the invention is shown as byway of an exemplary embodiment. FIG. 1 illustrates the prism 60according to this design in perspective view. By the edges 62, 64, 66and 68, represented in dashed lines extending in parallel in a pair-wiseand respectively group-wise manner, there is shown a regular prism bodyhaving triangular lateral faces 69. (Depicted in this embodiment is thespecial case of a isosceles rectangular triangle.) The continuous lines70 and 72 indicate those edge contours which delimit the light incidencesurface 74 and the light exit surface 76 (apart from the delimitingedges shown in continuous lines at 66 and 64). These two surfaces 74,76extend from the base surface 77 of the prism. It can be seen that thelight incidence surface 74 is inclined relative to the axis 79 by anangle of inclination 78 while the light exit surface 76 is inclined byan angle 80 of the same amount but oppositely to the inclination oflight incidence surface 74. This is also shown in the lateral views ofFIGS. 2 to 4. In the present exemplary embodiment, the angle ofinclination of both surfaces 74,76 is two angular degrees but can alsobe larger while, however, in the latter case the width 82 of the prism60 would have to be enlarged so as to still be able to conduct andcapture the light beam across it's the whole aperture of the prism.

The opposing inclination of the light incidence and exit surfaces 74,76is favorable for minimizing the disturbances caused by back reflectionwhich occur particularly under such angles of inclination of the prism60 where the light beam is incident vertically at least on one of thetwo surfaces. The reflections generated on the light incidence andrespectively exit surfaces 74,76 which act like mirror surfaces will bedeflected oppositely to the direction of incidence and (slightly) to theside, i.e. they will not travel back to the transmitter, which is to saythey will travel at an acute angle to the direction from which theincident rays come. Thus, the reflected beams can be masked out byoptical elements such as e.g. apertures or the like.

1. A prism, particularly for optical data communication by means of amodulated light beam, comprising a transmissive prism body which has twotriangular first side faces on its outer side and, between said firstside faces, a base surface and two second side faces rising up from thebase surface and inclined toward one another as light incidence andlight exit faces, said second side faces being inclined oppositely,relative to an axis running parallel to the base surface and penetratingthe planes in which the first side faces lie, and each running at anacute angle to the axis in such a manner that reflection radiation isreflected to the side.
 2. The prism according to claim 1, characterizedin that wherein the second side faces are anti-reflective.
 3. The prismaccording to claim 1, wherein the respective acute angle of inclinationis in the range from a few 1/10 angular degrees to a few tens of angulardegrees.
 4. The prism according to claim 1, wherein the second sidefaces, when viewed in the projection onto one of the first side faces,extend at an angle of 90°.
 5. The prism according to claim 1, whereinthe respective acute angle of inclination is in the range from anangular degree of 0.1 to 15 angular degrees
 6. The prism according toclaim 1, wherein the respective acute angle of inclination is in therange from 2 angular degrees to 5 angular degrees.
 7. The prismaccording to claim 4, wherein the second side faces, when viewed in theprojection onto one of the first side faces, have the same length.